bgpd: add vty command to select label allocation per nexthop

bgpd: add vty command to select label allocation per nexthop

A new VTY command is introduced in ipv4 unicast and ipv6 unicast
address family, under a BGP instance.

 > r1# label vpn export allocation-mode per-nexthop|per-vrf

This command will update the label values associated for each
BGP update to export to the global instance. Two modes are
available: per-nexthop and per-vrf. The latter is the default
one.

With this commit only, configuring label allocation per nexthop
will only reset the BGP updates, and the per-vrf mode label
allocation will be chosen.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>